1 files changed, 20 insertions, 20 deletions

diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 98afeb8cc5..c114ea0c0d 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -99,8 +99,15 @@ module ActiveRecord
       end
 
       def merge_multi_values
-        relation.where_values = merged_wheres
-        relation.bind_values  = merged_binds
+        lhs_wheres = relation.where_values
+        rhs_wheres = values[:where] || []
+        lhs_binds  = relation.bind_values
+        rhs_binds  = values[:bind] || []
+
+        removed, kept = partition_overwrites(lhs_wheres, rhs_wheres)
+
+        relation.where_values = kept + rhs_wheres
+        relation.bind_values  = filter_binds(lhs_binds, removed) + rhs_binds
 
         if values[:reordering]
           # override any order specified in the original relation
@@ -123,34 +130,27 @@ module ActiveRecord
         end
       end
 
-      def merged_binds
-        if values[:bind]
-          (relation.bind_values + values[:bind]).uniq(&:first)
-        else
-          relation.bind_values
-        end
-      end
-
-      def merged_wheres
-        rhs_wheres = values[:where] || []
-        lhs_wheres = relation.where_values
+      def filter_binds(lhs_binds, removed_wheres)
+        return lhs_binds if removed_wheres.empty?
 
-        if rhs_wheres.empty? || lhs_wheres.empty?
-          lhs_wheres + rhs_wheres
-        else
-          reject_overwrites(lhs_wheres, rhs_wheres) + rhs_wheres
-        end
+        set = Set.new removed_wheres.map { |x| x.left.name }
+        lhs_binds.dup.delete_if { |col,_| set.include? col.name }
       end
 
       # Remove equalities from the existing relation with a LHS which is
       # present in the relation being merged in.
-      def reject_overwrites(lhs_wheres, rhs_wheres)
+      # returns [things_to_remove, things_to_keep]
+      def partition_overwrites(lhs_wheres, rhs_wheres)
+        if lhs_wheres.empty? || rhs_wheres.empty?
+          return [[], lhs_wheres]
+        end
+
         nodes = rhs_wheres.find_all do |w|
           w.respond_to?(:operator) && w.operator == :==
         end
         seen = Set.new(nodes) { |node| node.left }
 
-        lhs_wheres.reject do |w|
+        lhs_wheres.partition do |w|
           w.respond_to?(:operator) && w.operator == :== && seen.include?(w.left)
         end
       end